1. Field of the Invention
This invention relates to a device for measuring electrical quantities wherein measurement of electrical quantities such as the current or power of the system under measurement is effected using a Hall element.
2. Description of the Related Art
If the applied magnetic field in a Hall element is made proportional to the current value of a system under measurement, a Hall voltage will be obtained that is proportional to this applied magnetic field and hence to the current value of the system under measurement. Thus a current measurement device can be constructed using such a Hall element. FIG. 4 shows an equivalent circuit of such a Hall element. In this FIG. 1 is a Hall element, T1 and T2 are a pair of current input terminals, and T3 and T4 are a pair of voltage output terminals. Equivalent resistances Ra, Rb, Rc and Rd are respectively formed between, adjacent terminals of these terminals T1-T4. B designates an applied magnetic field. However, if, when magnetic field B is not applied an offset voltage is present between voltage output terminals T3 and T4, this gives rise to a measurement error when a current measurement device or the like is constituted using Hall element 1. The offset is generated when equivalent resistances Ra and Rb are not equal to or equivalent resistances Rc and Rd are not equal to during when magnetic field B is not applied. Consideration has therefore been given to adjusting one of these four equivalent resistances Ra, Rb, Rc and Rd from outside of Hall element 1 such that the offset is reduced to zero, by a well-known method such as a variable resistor is used or a voltage is applied to Hall element 1. One example of compensating the offset is disclosed in Japanese Patent Disclosure (Kokai) No. H6-174765 published on Jun. 24, 1994. For example, using a Hall element 1 equipped with gates G1-G4 as shown in FIG. 5, one equivalent resistance, e.g. Rc, can be controlled by applying a voltage to one of four gates G1-G4. In this case, the usual conventional method was to leave the remaining three equivalent resistances Ra, Rb and Rd without exercising particular positive control over them. An input resistance Rin of Hall element 1 was then Rin=(Ra +Rc).parallel.(Rb+Rc), wherein the symbol ".parallel." means the parallel connection.
Previously there was the problem that when fluctuation of the offset occurred due to variation with time or temperature, since compensation was effected by making the offset zero by varying a single equivalent resistance, fluctuation in the input resistance occurred, which produces fluctuation of sensitivity and tends to give rise to measurement error.